The way in which an object reflects light can vary from perfectly diffuse, known in the art as Lambertian (after Lambert), to perfectly specular (after speculum, a mirror). If an object is substantially Lambertian in nature in that the surfaces reflect light with an efficiency which is essentially independent of angle, then the illumination of such an object is relatively simple. In such a case, the uniformity of an image of an object relies only upon the uniformity and intensity of the incident illumination. An example of a Lambertian object would be paper, which can be adequately illuminated by a single point-like source of light.
If an object is substantially specular and the desired illumination is bright field illumination, then the light source will be seen directly by the observer. This can be accomplished by placing a camera at an off angle which is the same as the off angle of a light source in so much as the angle of reflection on a specular object complements the angle of incidence. In such a case the source itself must have the characteristics of a Lambertian emitter and must encompass the projected field of view.
Between substantially diffuse reflecting Lambertian objects and substantially specular reflecting object there exists a very large class of objects for which the surfaces are neither substantially Lambertian nor substantially specular. For these objects, the amount of light reflected from a light source to the observer or sensing device depends both on the intensity of the incident illumination and the angle of incidence.
Designing a lighting system for imaging an object entails directing a light in a specific direction, generally toward the object being imaged. The usual way to accomplish this is to point or aim a light source in the direction of the object. This aiming can be quite involved. For example, U.S. Patent Application Publication No. 2004/0141175, filed as U.S. patent application Ser. No. 10/616,548 on Jul. 10, 2003 by Baldwin et al., presents an example of aiming light sources such as light emitting diodes (LEDs) from different directions to achieve uniform illumination of an object at a substantially constant angle of illumination regardless of the location of the object.
Darkfield ringlights are generally used to illuminate objects from a plurality of directions. A darkfield ringlight produced using state-of-the-art techniques includes LEDs incorporating a rudimentary condensing lens mounted to four, for example, individual circuit board segments such that the LEDs may be angled toward the object from different directions. Each circuit board segment is mounted at a suitable angle and positioned such that its energy is directed toward the object. The principal rays of the LEDs converge at a predetermined location. Although this arrangement provides adequate illumination, there are difficulties associated with its implementation. For example, it is expensive to produce, mount and interconnect four separate circuit board assemblies.